AJP - Heart and Circulatory Physiology, Vol 256, Issue 2 391-H403, Copyright © 1989 by American Physiological Society
Effects of tissue geometry on initiation of a cardiac action potential
R. W. Joyner, B. M. Ramza, R. C. Tan, J. Matsuda and T. T. Do
Todd Franklin Cardiac Research Laboratory, Department of Pediatrics, Emory University, Atlanta, Georgia 30322.
We used rabbit ventricular papillary muscles and isolated rabbit
ventricular muscle cells to compare the effects of a decrease in cardiac
excitability. For the papillary muscles, we defined tissue excitability as
the inverse of the current required to initiate a propagated action
potential from a local stimulus. For the isolated cells, we defined
cellular excitability as the inverse of the current required to initiate a
membrane action potential. For papillary muscles, lidocaine with elevated
extracellular K+ concentration ([K+]o) decreased maximum rate of rise of
membrane potential (Vmax), decreased conduction velocity, and strongly
decreased tissue excitability. For the isolated cells, lidocaine with
elevated [K+]o decreased Vmax but had little effect on cellular
excitability. We interpret our results on the differences of effect on
tissue excitability vs. cellular excitability as a consequence of the
syncytial nature of the papillary muscle. The cell-to-cell electrical
connections produce an electrical load on the locally stimulated region.
This electrical load makes the tissue excitability dependent on the amount
of inward current that the locally excited cells and the surrounding cells
can generate. We simulated these phenomena with numerical solutions of
action potential initiation in an isopotential cell compared with a
two-dimensional disk of excitable tissue. The simulation results recreate
the basic experimental observation that the sensitivity of the current
threshold to agents that lower inward current is markedly larger for
multidimensional current flow from a source compared with an isopotential
system.
